The present invention relates to a transparent substrate, in particular a glass substrate, provided with a thin-film stack including at least one metal film with properties in the infrared, in particular a low-emissivity one, arranged between two coatings which each include at least one anti-reflection film based on dielectric material, and at least one protective film based on a sacrificial metal or a partially or fully oxidized sacrificial metal or a sacrificial metal alloy placed between the said metal film and one of the said coatings. According to the invention, the film placed directly below and/or above the said metal film contains at least one of the noble metals Pd, Au, Ir, Pt and Rh.
The thin-film stacks with this structure are described in a number of embodiments. They are deposited on an industrial scale, usually in continuous devices, on polymer sheets or glass panes, using the method of magnetic field-enhanced cathodic sputtering. Dielectric anti-reflection films consist mainly of metal oxide films which are sputtered from a metal target reacting in a working gas containing oxygen. The protective film placed between the silver film and the anti-reflection cover film therefore has the purpose, on the one hand, of protecting the underlying silver film, during the reactive sputtering of the anti-reflection cover film, from the oxidizing effect of the oxygen (EP 0 104 870 B1, DD 288 822) and also, on the other hand, of improving the long-term stability of the thin-film stack with regard to atmospheric attack (pollution, etc.).
In addition, document EP-0-433 136 B1 discloses a laminated heated window for a motor vehicle, including a pane covered with a multifilm which serves as a heating element and which comprises a conductive. silver film and a protective zinc oxide film which reduces reflection and, furthermore, on this protective film, a thin film of nickel-chromium or tantalum or an oxide of these metals which improves the stability of the multifilm and also improves the adhesion of the multifilm with which the pane is equipped to the thermoplastic insert.
It is also known that, in order further to improve the long-term stability of thin-film stacks of this type with regard to atmospheric attack, as well as the corrosion durability behaviour of the film, and the stability with regard to elevated temperatures, a protective film of a sacrificial metal or a sacrificial metal alloy is also deposited below the silver film (DE 4 109 708 C1).
The sacrificial metals found as protective films not only above, but also below the silver film are basically metals or metal alloys which have a relatively high affinity for oxygen and which bind with the oxygen which has diffused into these protective films, with the formation of the corresponding metal oxides. Because the protective films in the metallic state reduce the transmission of the thin-film stack, these protective films are preferably deposited in a thickness such that they are oxidized more or less completely during the subsequent reactive sputtering of the anti-reflection film and/or during a later heat treatment. Their protective effect relating to the corrosion durability behaviour of the thin-film stack is also obtained in their at least partially oxidized form.
Document DD 288 822 describes the sputtering of a protective metal film placed above the silver film, from a TiPd alloy target, where the quantity of Pd in the alloy is to be 0.1 to 1.0%. Using a protective alloy film of this type, the resistance to moisture and temperature as well as the long-term stability of the thin-film stack are increased.
Similarly, it is known that, in order to improve the stability of a thin-film stack of this type with regard to moisture and with regard to chemical attack, the silver film is protected on one or both of its faces by a special barrier agent, which consists of a double film with a first partial film of Pt and/or Pd and a second partial film of Ti and/or Cr, or of a single film made of an alloy with at least 15 atom % of one of these elements. If the barrier agent consists of two separate films, pure Pt and/or Pd targets must be used for its preparation, which entails high costs. The partial films of pure Pt and/or Pd also have the effect of lowering the transmission of the thin-film stack. The favourable effect of these known thin-film stacks, relating to chemical stability, relies on the presence of the noble metal film on the silver film, by virtue of which the latter is protected against extraneous attack.